A. Technical Field
The present invention relates generally to the field of magnetic field sensors and more particularly to method of noise and offset reduction in magnetic field sensors.
B. Background of the Invention
Magnetic field sensors have been commonly used in various electronic devices, such as computers, laptops, media players, smart phones, etc. There are several techniques/devices that can be used for detecting a magnetic field. Tunneling Magnetoresistance (TMR) is a promising magnetic sensing technology for handset applications due to its advantages in sensitivity, power, and process cost compared with other magnetic sensors. Another closely related technology in magnetic field sensing is Giant Magnetoresistance (GMR).
A TMR element is composed of two ferromagnetic layers separated by a non-magnetic, insulating tunnel barrier. One layer has a magnetization direction that is “free” to rotate in a magnetic field. The other layer has a “fixed,” reference magnetization that does not rotate when in a magnetic field of moderate to low strength that is of sensing interest. If the magnetization directions of the two layers are parallel to each other, the electrical resistance of the tunnel barrier is low. Conversely, when the magnetization directions are anti-parallel, the resistance is high. A magnetic field sensor based on TMR therefore converts magnetic field into electrical signal by a change in electrical resistance due to the changing angle of the magnetic free layer relative to the fixed layer in response to the field.
In this type of sensor, a magnetic 1/f noise is caused by fluctuations in the magnetic orientation of regions in one or both of the layers. These fluctuations create changes in resistance at different time scales that appear as noise in the target signal and cannot be simply filtered out. A 1/f noise (also called pink noise) is a signal or fluctuation process with a frequency spectrum density (energy or power per Hz) inversely proportional to the frequency of the signal. In addition, a zero-field offset can also be created by differences in the magnetic orientation of regions in either magnetic film, which may be dependent on temperature or stress in the sensor.
Therefore, it would be desirable to have a system, device and method to effectively reduce 1/f noise and offset in TMR magnetic field sensors.